Times parameter description for solar position algorithms

[AdamRJensen]

pvlib currently uses variants of the following description for the times parameter in the solar position algorithms:

times : pandas.DatetimeIndex
    Must be localized or UTC will be assumed.

However, times should actually be UT1!

UTC has been maintained within ±0.9 of UT1 since 1970 ish by adding leap seconds. However, prior to 1970s there are large deviations between UTC and UT1, and there may be in the future too. Therefore, I think it's wise to update the times parameter to something along the lines of:

times : pandas.DatetimeIndex
    Must be localized or UTC/UT1 will be assumed. Prior to 1970 and far in
    the future UTC and UT1 may deviate significantly and in such cases
    UT1 times should be provided.